Magnetic resonance methods and apparatus



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NICK A. SCHUSTER HIS ATTORNEY March 26, 1963 Filed Oct. 5, 1955 N. A.SCHUSTER MAGNETIC RESONANCE METHODS AND APPARATUS 4 Sheets-Sheet FILTERREJECT f GATED AME l DETECTOR FIG. 3

NICK A. SCHUSTER 'QlCJW HIS ATTORNEY March 26, 1963 N. A. SCHUSTERMAGNETIC RESONANCE METHODS AND APPARATUS 4 Sheets-Sheet 3 Filed Oct- 5,1955 m 0; 1 m 0- u A v v A H NM} n 3 n H B.EQ LI 1 3252mm u 1 N22. 5 owf oh no 1 on on 8 8 A A 18.55 -o E55 E 025551 6 r 1 If. |||l|l|||||||||J mm mm 1 mi :831 B @9350 55w ESE s m I; n E Fills... l l I Ill l s z a vl 8 5 8 @m I f Nm 5 INVENTOR.

NICK A. SCHUSTER BYE I HIS ATTORNEY March 1963 N. A. scHusTER 3,083,335

MAGNETIC RESONANCE METHODS AND APPARATUS Filed Oct. 5, 1955 4Sheets-Sheet 4 46 4? 46 7 c K g E W FIG. 7 INVENTOR.

NICK A. SCHUSTER ms ATTORNEY Q 3,083,335 Patented Mar. 26, 19533,083,335 lhIiaGNE l IC RESONANCE METHODS AND APPARATUS Nick A.Scbustcr, Ridgefield, Conn., assiguor, by means assignments. toSchiumberger 'l'l'ell Surveying Corporation, Houston. Tern, acorporation of Texas Filed (let. 5, 1955, Scr. No. 538,578 14 Claims.(1. 324.5)

The present invention relates to magnetic resonance methods andapparatus, and more particularly to new and improved methods andapparatus for detecting magnetic resonance phenomena in particlesexhibiting paramagnetic properties and which may be particularly useful,for example, in the non-destructive chemical analysis of materials.

Nuclear and other paramagnetic resonances have been observed in the pastas described for example by Bloembcrgen, Purcell, and Pound in anarticle entitled Relaxation Effects in Nuclear Magnetic ResonanceAbsorption, Physical Review, vol. 73, page 679 (1948), by Bloch, Hansenand Packard in an article entitled The Nuclear Induction Experiment,Physical Review, vol. 70, page 474 (1946), and b E. L. Hahn in anarticle entitled Spin Echoes, Physical Review, vol. 88, page 589 (1950).In general, a sample containing paramagnetic particles is placed in aconstant magnetic field that is substantially homogeneous throughout thesample. An alternating magnetic field is applied to the sampleperpendicularly to the constant magnetic field and at a frequencysubstantially equal to the resonance precession frequency of theparamagnetic particles in the constant field. Under these conditions themagnetic moments associated With particular particles will process atthe resonance precession frequency, and this resonance is observed inone of the following ways: (I) as a result of the absorption ordispersion of the applied alternating magnetic field; (2) as a result ofthe voltage induced by the magnetic field generated at the precessionfrequency by the precessing magnetic moments; or (3) by modulatingeither the constant magnetic field or the applied alternating magneticfield, or both, and observing the effects of the modulation on theresonance signal detected in accordance with either of the first twomethods above. It should be noted that regardless of the observationmethod employed, the resonance is detected at the resonance precessionfrequency which is also the frequency of the applied alternatingmagnetic field.

Such methods have been successful in the carefully controlled eonditionsof a laboratory where time and care may be taken to distinguish theresonance signal from the far larger applied alternating magnetic fieldof the same frequency. In addition, in the laboratory both the constantand the applied alternating magnetic fields may be made almostcompletely homogeneous throughout the sample, whereby the resonancesignal i maintained at a relatively high intensity. However, forcommercial reasons it is not always possible to use laboratory time andcare to detect resonance. In addition, Where the magnetic fields cannotfor practical reasons be made homogeneous, the resonance signal will beof relatively low intensity, and since this signal is at the samefrequency as the applied alternating field, the small resonance signalmay be ditficult to detect in the presence of the relatively largealternating field at the same frequency.

Accordingly, it is a primary object of the present invention to providenew and improved methods and apparatus for detecting magnetic resonancephenomena which are of particular utility where high degrees of fieldhomogeneities are not practical.

Another object of the present invention is to provide new and improvedmagnetic resonance detecting methods and apparatus in which theresonance is detected at a frequency substantially different from theresonance precession frequency.

These and other objects of the invention are attained by applying analternating magnetic field substantially perpendicularly to a constantmagnetic field within a sample containing paramagnetic particles. Thefrequency of the alternating magnetic field is selected substantiallyequal to the resonance precession frequency of the particles in theconstant magnetic field. The alternating magnetic field is modulated andthe effect of this modulation on the constant magnetic field isdetected, i.e., the variations in the magnetic field which issubstantially perpendicular to the processing components of the magneticmoments are detected. Substantially no signal at the resonance frequencyis induced in the direction of the constant magnetic field since boththe applied and the induced alternating magnetic fields areperpendicular thereto thus simplifying the detection of variations insaid constant field resulting from resonance. Further, the resonance isdetected at a frequency which is dependent on the amplitude of theapplied alternating magnetic field and which is selected to besubstantially different from the resonance precession frequency.

In one typical embodiment the alternating magnetic ficld is appliedperiodically in the form of pulses of predetermined intensity. Therelaxation signal is detected at a frequency which is proportional tothe selected in tensity of the applied alternating field. In a secondtypical embodiment the alternating magnetic field is appliedcontinuously and i5 amplitude-modulated. The resonance is observed bydetecting the variations in magnetic susceptibility resulting from themodulation. In a third typical embodiment the alternating magnetic fieldis applied in the form of pulses which change the magneticsusceptibility, and this change is detected in the intervals betweenpulses. An arrangement for utilizing the principles of the presentinvention in the logging of earth formations is described.

The invention will be more fully understood with reference to theaccompanying drawings in which:

FIG. 1 illustrates typical apparatus for carrying out one embodiment ofthe invention;

FIG. 2 is a series of pulse diagram utilized to explain the operation ofthe apparatus shown in FIG. 1;

FIG. 3 illustrates a modification of the apparatus shown in FIG. 1 forincreasing the signal-to-noise ratio;

FIG. 4 is a series of pulse diagrams utilized to explain the operationof the apparatus shown in FIG. 3;

FIG. 5 illustrates a typical form of apparatus for carrying out a secondembodiment of the invention;

FIG. 6 illustrates a form of apparatus for carrying out a thirdembodiment of the invention;

FIG. 7 is a pulse diagram utilized to explain the operation of theapparatus shown in FIG. 6; and

FIG. 8 represents apparatus construction in accordance with theinvention suitable for making measurements of the formations traversedby a borehole.

In the apparatus shown in FIG. 1 a sample 10 containing paramagneticparticles having a gyromagnetic ratio 1 is, for ease in explanation,shown placed between the pole faces of a permanent or electromagnetic 11which generates a substantially constant magnetic field parallel to theaxis 2 and has an average intensity H throughout paramagnetic sample 10.However, it will be understood that since the present invention does notrequire the high degree of field homogeneity heretofore necessary, itwill find its greatest utility in applications where the material to beinvestigated cannot actually be placed between the pole faces of amagnet. A coil 12 is mounted to have its longitudinal axis xperpendicular to the z axis through sample and is adapted to beenergized by current from R.F. current generator 13.

In this embodiment of the invention generator 13 may comprise acrystal-controlled oscillator 14 generating current of frequency f whichis coupled to coil 12 through an amplifier 15 adapted to be gated bypulses from timing circuit 16 via conductor 17. The frequency i isselected substantially equal to the resonance precession frequency ofthe paramagnetic particles in the constant field of average intensity Hwhere Thus when gated amplifier 15 is closed by pulses 18 (FIG. 2) ofduration 1 RF. current 20 of predetermined intensity is passed throughcoil 12. This current passing through coil 12 generates an alternatingmagnetic field of average intensity 2H throughout paramagnetic sam ple10. The significant component of this alternating field has an intensityH effectively rotating about the z axis in the x, y plane at the angularfrequency w :21rf As is well-known, the other component of intensity His effectively rotating in the opposite direction at the angularfrequency m and may thus be disregarded.

As shown in FIG. 2, after an interval r the gated amplifier 15 is openedfor a period 7 during which time the alternating magnetic field atfrequency 01 (hereinafter referred to as H is not applied to sample 10.The periods 1' and r may thereafter be repeated indefinitely. In thisfirst embodiment of the invention the periods 1'; should be no longerthan T,* where and where AH is the half-width of the inhomogeneity infield H throughout that portion of sample 10 under investigation. Theperiods r should be at least as long as the relaxation time T associatedwith the precession. Under these circumstances, an alternating magneticfield of frequency f will exist along the z axis superposed on theconstant field perpendicular to activating coil 12, during the intervalswhere This alternating magnetic field will induce a signal 21 (FIG. 2)at frequency f; in a coil 22 having its longitudinal axis parallel tothe z axis.

The output of coil 22 may be connected to any suitable detectingapparatus 23. For example, as shown in FIG. 1, detecting apparatus 23may comprise a filter 24 adapted to reject any signal at the resonancefrequency f induced in coil 22 from coil 12 and to pass any signal offrequency f The rejection of frequency t is desirable for, while coil 22is substantially perpendicular to both the induced and appliedalternating fields of frequency f it is not possible to eliminateentirely frequency f from the output of coil 22. The output of filter 24at frequency f may be amplified by amplifier 25 and detected by detector26, the output of which may be connected to a meter 27 indicatingresonance. To increase the signal-to-noise ratio, amplifier 25 may be ofthe gated type so that the detecting apparatus is activated only duringthe periods T1, by the connection of amplifier 2 5 to timing circuit 16via conductor :19.

As can be seen from relations (1) and (3) above, by properly selectingthe intensity of the current through coil 12 so as to make H appreciablydifferent from H frequency will be distinctly different from frequency fAccordingly, the resonance signal may be easily separated from anyinduced signal of frequency f by means of detecting apparatus 23.

In a typical example adapted for the detection of nuclear magneticresonance, assume it is desired to detect hydrtv gen (proton) resonancein sample 18, where and T =l sec. If H =25O gauss, from relation (1)above, f should be approximately equal to 1.06 me.-

If H is selected equal to 20 gauss, then f will be equal to kc. If T l0-sec., then T; may be made equal to V2 X10" sec., while T is selectedequal to 1 sec.

A modification of the apparatus shown in FIG. 1 to incorporate some ofthe principles disclosed in my copending application Serial No. 463,776filed October 21, 1954 for: Magnetic Resonance Methods and Apparatus.now Patent No. 2,968,762 issued January 17, 1961, is shown in FIG. 3,where elements having similar functions to elements shown in FIG. I bearthe same numerical designations. In accordance with the principles setforth in said copending application, the duration of interval 1"; may beincreased relative to the duration of the interval 7 thereby increasingthe time during which measurements are made, and thus increasing thesignal-to-noise ratio. T 0 this end the phase of the alternating currentactivating coil 12 is periodically reversed during the interval T1. Toaccomplish this, the output of oscillator 14 may be split between twogate circuits 28 and 29 which, when closed, give respectively oppositelyphased current into the input of amplifier 33.

Thus when gate 28 is closed by a pulse from timing circuit 30, currentof one phase at frequency t passes through the coil 12 while when gate29 is closed by a pulse from timing circuit 30, current of frequency fbut of opposite phase passes through coil 12. In a typical example,during the interval 1 (FIG. 4) gate 25 is first. closed for a period ofduration such that Gate 28 is then opened and gate 29 closed for aperiod t where 1111 (known as a -41r pulse in said copendingapplication). Gate 29 is then opened and gate 28 closed for a period 2;(+4"), followed by a period 1 during which gate 29 which is closed(-41r) and terminated by a period of duration t during which gate 28 isclosed (+21). This sequence is shown in FIG. 4B.

The detecting apparatus 23 may be identical to the apparatus disclosedin FIG. 1, the gated amplifier 25 being closed for the entire period 7It will be understood that the +21!" -41r- +4'rr' -41r, +211' sequencediscussed in connection with FIG. 3 is merely exemplary and the othersequences suggested in said copending application may be employed,except those in which the resonance is to be measured during a freeprecession interval when the moments are substantially perpendicular tothe constant magnetic field.

The embodiments described in connection with FIGS. 1 and 3 may beconveniently combined with other detecting methods to further increasethe signal-to-noise ratio. For example, in circuits shown in FIGS. 1 and3 a second detecting coil may be placed along the y axis perpendicularlyto the z and .r axes and the resonance signal induced at the resonanceprecession frequency detected therein in the usual manner. This inducedsignal may be combined with the one detected by detecting apparatus 23,thereby increasing the signal-to-noise ratio. In connection with theapparatus described with respect to FIG. 3, the circuits described inconnection with the aforementioned copending application may be used todetcct the resonance in addition to that shown in FIG. 3 and the tworesonance signals detected and combined. In the apparatus shown in FIG.1, coil 12 may form a portion of a bridge circuit utilized to detect theabsorption of energy at frequency i in coil 12 and this absorptionsignal combined with the resonance signal detected in the output of coil22 to increase the signal-to-noise ratio.

In FIG. 5 there is shown an embodiment in which the alternating magneticfield is continuously applied. In order to better understand theembodiment shown in FIG. 5, consider a region within sample 10 in whichthe in tensity of the field generated by magnet 11 is actually H If thisregion contains, for example, hydrogen nuclei, this region will have aCurie susceptibility (magnetic susceptibility at zero frequency) givenby where K is a constant, N is the number of hydrogen nuclei in sampleIt and T is the temperature. If now the field H at frequency I is turnedon by applying alternating current to coil 12, transitions will beinduced between the two possible magnetic dipole states and thepopulation levels of the two states will tend to equalize after a periodof time dependent on the relaxation time T. As the population levels ofthe two states tend to become equalized. the magnetic susceptibility Xof sample is reduced. in other words, from relation (4) above energycoupled from coil 12 to sample 10 raises the effective temperature" T ofthe nuclei. thereby reducing X By modulating in amplitude the field H ata relatively low frequency, the magnetic susceptibility along the z axiswill be modulated in amplitude, thus producing a modulation of the totalflux linking coil 22. This signal will be of constant frequency whichwas the frequency of the modulation of H Thus in FIG. 5 the output ofoscillator 14 at frequency i is amplitudeunoduluted in modulator 31 by asignal e of frequency i generated in low frequency source 32 and appliedto coil 12 through amplifier 33. The field H is thus continuouslyapplied by coil 12 and is modulated in amplitude by signal This in turnmodulates the magnetic susceptibility along the z axis at frequency fThe signal along the z axis at frequency i picked up by coil 22 may bepassed through filter 24 which rejects any signal of frequency famplified by amplifier 34, detected by detector 26 and indicated onmeter 27 as a function of the resonance within sample it). As a typicalexample, the intensities of H and H may be the same as described inconnection with FIG. 1, with the signal e providing modulation in therange from 50 to 100% at a frequancy f such that l/f is approximatelyequal to the relaxation time T However, in the cases where therelaxation time T is extremely short, for example in electronparamagnetic rcsonanccs, the periods 1/1 may of practical necessity bemade longer than the time T Also, in the cases where the relaxation timeT is relatively long, as in the case of proton resonance, it may bedesirable to decrease the period l/f since electronically it is oftendifficult to accurately detect signals at a very low frequency, e.g.,one cycle/second.

in addition, the apparatus shown in FIG. 5 may be utilized to measurethe relaxation time T of the resonance phenomenon. To this end theoutput of amplifier 34 may be applied to a limiter 35 having a constantoutput regardless of the intensity of the resonance signal picked up incoil 22. The phase of the output of limiter 35 and thus the phase of theresonance signal detected in coil 22 is compared to the phase of themodulation envelope modulating the current through coil 12. To this endoutput limiter 35 is applied to phase sensitive detector 36 sensitizedby a reference signal from generator 32 via conductor 37. The variationsin the output of the phase sensitive detector indicate the variations inphase of the signal detected in coil 22 as the sample 10 is movedrelative to magnet 11. These variations are a function of the variationsof the relaxation time T and may be indicated on a meter 38. Asdisclosed in my copending application Serial No. 330,978 filed January13, 1953, for Analysis of Substances by Measurement of NuclearRelaxation Time, the measurement of T is indicative of the particularcompound in which the paramagnetic particles under investigation arefound. Thus the apparatus shown in FIG. 5 may be utilized to determinethe presence of particular paramagnetic particles and to indicate theparticular compound containing said particles.

While the signal e is indicated in connection with FIG. 5 as being asine wave, it may be a square Wave, providing for example modulation InFIG. 6 is shown additional apparatus for measuring relaxation time inaccordance with the present invention. Current at frequancy f is appliedto gated amplifier 15. Periodically amplifier 15 is closed by pulses 39(FIG. 7A) on conductor 40 from timing circuit 41. The duration of thesepulses is approximately equal to 1r/'yH where H is the effectiveintensity or the alternating field applied through sample 10 by coil 12.Thus the alternating field is applied for periods of duration just longenough to nutate the macroscopic moments previously aligned with field Hby 1r radians (a Tr pulse) or to a position opposite to the direction ofthe constant magnetic field. Between pulses 39 occur periods 42 duringwhich no alternating field is applied and during which the moments areallowed to relax back into line with the constant field. This causes aneffective modulation of the magnetic susceptibility as shown by thecurve 43 (FIG. 7B) which increases during periods 42 exponentially as afunction of relaxation time T The intensity H is made as large aspractical so that the pulses 39 may be as short as possible. In anyevent, the duration of pulses 39 must be short with respect to theanticipated relaxation time T of the particles under investigation. Itshould be noted that even though field H is not homogeneous, any momentswhich are nutatcd out of alignment with the direction of the field Hwill contribute to the effect, and those moments which are nutatedbetween 1r/2 and 31r/2, for example, will contribute substantially tothe effect.

Alternatively, after the macroscopic moments are aligned with the fieldH in one direction, the direction of this field may be reversed in atime short with respect to the relaxation time T Thus the moments willbe substantially 180 out of line with the field H In the intervals ofduration at least equal to T between field reversals. the moments areallowed to relax back into line with the constant field H therebymodulating the magnetic susceptibility. The field H may be reversed byutilizing an electromagnet to generate H and periodically reversing thedirection of the direct current passing therethrough. In thisarrangement no applied alternating field (H is required. The detectingcircuit is preferably opened during the times when the field is beingreversed.

The variations in magnetic susceptibility along the z axis may bedetected in any known manner. For example, as shown in FIG. 6, the fieldvariations as detected by coil 22 may be applied to filter 24 and thencein parallel to gated amplifiers 44 and 45. The gating of theseamplifiers is timed to sample curve 43 at two distinct times during eachcycle. For example, shortly after each pulse 39, amplifier 44 is closedby pulses 46 sampling the amplitude of curve 43 at point 46'; thenamplifier 45 is closed by pulses 47 at a later time, giving sample 47'.Samples 46 and 47' are detected by detectors 48 and 48 respectively.Their intensities are compared by amplitude comparator 49 and thedifference indicated by meter 27. This difference is indicative of therelaxation time T In the previous embodiments coil 22. is shown in theinterval between pole faces. It wil be appreciated that this coil may beanywhere in the constant magnetic circuit so as to detect variationstherein. In addition, other known types of magnetic field sensingelements may be employed.

The apparatus described in the foregoing embodiments are extremelyuseful, for example, in the non-destructive chemical analysis ofmaterials. For example, the previous embodiments may be convenientlyutilized in connection with the logging of the formations traversed by aborehole. Typically, as shown in FIG. 8 a pressureresistant non-magnetichousing 50 is adapted to be raised and lowered through a borehole 51 bymeans of an electrical cable 52 and winch (not shown) at the surface ofthe earth. One face 53 of housing 50 is adapted to be pressed againstthe wall of the borehole 51 continuously by means of a spring -4. Withinthe upper portion 55 of housing 50 may be the necesary electronics andpower supply circuits to operate in accordance with the invention.Immediately below cartridge 55 there is an electromagnet 60 activated bycarefully regulated constant D.C. current through conductors 61 and 62.A detecting coil 63 wound parallel to the axis of magnet 60 has as itsoutput conductors 64 and 65. A coil 66 at right angles to the magneticfield produced by magnet 60 is utilized to generate the field H withinthe formation material.

In operation, housing 50 is lowered to the bottom of the lowermostportion of the borehole in which a log is desired. It is then raisedslowly. As housing 50 is raised, the earth material to be analyzed willpass within the infiuence of the field H generated in formations bymagnet 60 parallel to the vertical axis of the borehole. The constantfield generated by magnet 60 will decrease with the distance from thewall of borehole '50. If for example it is desired to detect resonancein the formation material between 6 inches and 12 inces from the wall ofthe borehole, coil 66 will be activated by a ban-d of frequencies in thespectrum between H,, at 6 inches through H at 12 inches.

This band of frequencies applied to coil 66 is modulated in amplitude bythe signal e as described in connection with FIG. 5. The variations inmagnetic susceptibility will appear in the output 6465 of coil 63, asexplained above, and may be utilized to determine the presence ofresonance and of the relaxation time of the materials involved.

Accordingly, contrary to the prior art, it is not necessary that anextremely homogeneous field be employed since the band of frequenciescan be employed to activate coil 66, the resonance signal being detectedat the frequency i In addition, the apparatus is made sensitive to thematerial Within a predetermined depth of the borehole, thus sensitizingthe equipment to the desired formation material and de-sensitizing it toundesired material. Obviously the other embodiments described above maybe used in connection with the borehole apparatus as shown typically inFIG. 8. For example, a reversing switch 63 may be connected between aregulated constant D.C. source and electromagnet 60 by conductors 61 and62 in order to reverse the field H in a time short with respect torelaxation time T of the material undergoing investigation. As describedabove, no applied alternating field (H is required in order to pick upthe signal due to relaxation by means of detecting coil 63. Instead,conductor 40 may be connected to supply pulses 39 to reversing switch 68(FIG. 8) so that the polarizing field is switched at a time when thegating amplifiers 44, 45 (FIG. 6) are closed to block signaltransmission to the detectors. In this case, of course, pulses 39 are ofa duration short with respect to the relaxation time T but need not beof the same interval 1r/7H Rather than change the connection ofconductor 40 from amplifier (FIG. 6) to reversing 8 switch 68 (FIG. 8),a two-pole switch 70 may instead be used for this purpose.

It will occur to those skilled in the art that the foregoing is subjectto many modifications. For example, the constant field H may be theearths magnetic field, in which event no permanent or electromagnet 11will be necessary. Thus the representative embodiments illustrated aboveare not considered as limiting the appended claims.

I claim:

1. Apparatus for observing magnetic resonance phenomena in particlesexhibiting paramagnetic properties wherein a substantially constantmagnetic field is applied to said particles comprising means forapplying an alternating magnetic field of effective intensity H to saidparticles during first time intervals, said alternating magnetic fieldeffectively rotating perpendicularly to said constant magnetic field atsubstantially the resonance precession frequency of said particles insaid constant magnetic field, said first time intervals being separatedby second time intervals of duration long with respect to said firsttime intervals, a filter for rejecting said precession frequency, andmeans including said filter and a detector coupled to the outputthereof, for detecting variations in said constant magnetic field duringsaid first time intervals at a frequency substantially equal to 'yH/21r, where 'y is the gyromagnetic ratio for said particles.

2. Apparatus for observing magnetic resonance phenomena in particlesexhibiting paramagnetic properties wherein a substantially constantmagnetic field is applied to said particles comprising means forapplying an alternating magnetic field to said particles during firsttime intervals, said alternating magnetic field efiectively rotatingperpendicularly to said constant magnetic field at substantially theresonance precession frequency of said particles in said constantmagnetic field, said first time intervals being separated by second timeintervals of duration long with respect to said first time intervals,means for periodically reversing the phase of said alternating magneticfield during said first time intervals, and means for detectingvariations in said constant magnetic field during said first timeintervals at a frequency proportional to the intensity of saidalternating magnetic field.

3. Apparatus for observing magnetic resonance phenomena in particlesexhibiting paramagnetic properties wherein a substantially constantmagnetic field is applied to said particles comprising means forapplying an alternating magnetic field to said particles effectivelyrotating perpendicularly to said constant magnetic field atsubstantially the resonance precession frequency of said particles insaid constant magnetic field, means for modulating the intensity of saidalternating magnetic field with a first signal of a second frequencysubstantially lower than said resonance precession frequency, means fordetecting a second signal proportional to the variations in saidconstant magnetic field at said second frequency, and means forcomparing the phase of said second signal with the phase of said firstsignal independently of amplitude variations to provide indications ofthe phase variations of said second signal with respect to said firstsignal.

4. Apparatus for observing magnetic relaxation phenomena in particlesexhibiting paramagnetic properties comprising means for applying apolarizing magnetic field to said particles, means for periodicallyreversing the direction of said polarizing magnetic field duringintervals short with respect to the relaxation time T associ ated withsaid particles, and means for detecting variations in the resultantmagnetic field in the vicinity of said particles which are a function ofmagnetic relaxation of said particles during the periods between saidintervals.

5. Apparatus for observing magnetic resonance phenomena in particlesexhibiting paramagnetic properties and having a gyromagnetic ratio 7wherein a substantially constant magnetic field is applied to saidparticles comprising means for applying an alternating magnetic field ofefiective intensity H and given phase to said particles during firsttime intervals of duration substantially equal to 1r/ 7H saidalternating magnetic field effectively rotating perpendicularly to saidconstant magnetic field at subtantially the resonance precessionfrequency of said particles in said constant magnetic field, said firsttime intervals being separated by second time intervals of duration longwith respect to said first time intervals, and means including a filterto block signals at said precession frequency for detecting variationsin said constant magnetic field during said second time intervals.

6. Apparatus for logging the formations traversed by a boreholecomprising a support adapted to be passed through said borehole, meansin said support for applying a substantially constant magnetic field tosaid formations, means in said support for applying an alternatingmagnetic field to said formations, said alternating magnetic fieldefi'ectively rotating perpendicularly to said constant magnetic field atsubstantially the resonance precession frequency of particularparamagnetic particles in a constant field of the intensity of saidconstant field in at least one location in said formations, means formodulating the intensity of said alternating magnetic field, and meansfor detecting variations in said constant magnetic field resulting fromsaid modulation.

7. Apparatus for logging the formations traversed by a boreholecomprising a support adapted to be passed through said borehole, meansin said support for applying a substantialiy constant magnetic field tosaid formations, means in said support for applying an alternatingmagnetic field to said formations, said alternating magnetic fieldeffectively rotating perpendicularly to said constant magnetic field ina band of frequency corresponding to the resonance precessionfrequencies of particular paramagnetic particles in a constant magneticfield having intensities of said constant field at a plurality oflocations in said formations, means for modulating the intensity of saidalternating magnetic field, and means for detecting variations in saidconstant magnetic field resulting from said modulation.

8. In a method of determining the magnetic resonance properties of thatportion of material located at a predetermined depth beneath the surfaceof said material, the steps of applying a substantially constantdivergent magnetic field between spaced-apart locations on said surfacewhereby said field decreases in intensity with the distance behind saidsurface, applying an alternating magnctic field perpendicularly to saidconstant field in said material and at a frequency equal to theresonance precession frequency of selected paramagnetic particles in aconstant field of the particular intensity of said constant field at thepredetermined depth of said portion, and obtaining indications of themagnetic resonance in said particles.

9. In a method of obtaining signals representative of magnetic resonancephenomena in particles exhibiting paramagnetic properties wherein asubstantially constant magnetic field is applied to said particles, thesteps of applying an alternating magnetic field of given phase to saidparticles substantially at right angles to said constant field andhaving a frequency substantially equal to the resonance precessionfrequency of said particles in said constant field, periodicallyreversing the phase of said alternating field after an interval given byn1r/27H where 'y is the gyroma netic ratio for said particles, H is theeffective intensity or said alternating magnetic field and n is aninteger between 1 and 4, and deriving a signal at the notationalfrequency of said particles during said periodic reversal responsive tovariations in the intensity of said constant field.

10. In a method of obtaining signals representative of magneticresonance phenomena in particles exhibiting paramagnetic propertieswherein a substantially constant f0 magnetic field is applied to saidparticles, the steps of applying an alternating magnetic field of givenphase to said particles substantially at right angles to said constantfield and having a frequency substantially equal to the resonanceprecession frequency of said particles in said constant field, amplitudemodulating said alternating field at a given relatively low frequency,detecting variations in said constant field at said given frequency,subjecting said substantially constant magnetic field and said particlesto relative movement, and deriving indications of the mag neticrelaxation time of said particles responsive solely to the dilference inphase of said detected variations and field modulation as said particlesand said constant field are subjected to relative movement.

11. A method for investigating earth formations traversed by a boreholeand containing particles exhibiting paramagnetic properties in thepresence of a magnetic field H applied to said particles, comprising thesteps of periodically reversing the magnetic field applied to saidparticles in successive earth formations to subject said particles tosuccessive magnetic fields in opposite directions during the timeintervals which are long relative to the time intervals for reversingsaid magnetic field, and during alternate time intervals detecting themagnetic relaxation signal resulting from relaxation of the macroscopicmagnetic moment associated with said particles toward alignment withsaid magnetic field.

12. A method for investigating earth formations traversed by a boreholeand containing particles exhibiting paramagnetic properties in thepresence of a mag netic field H applied to said particles, comprisingthe steps of periodically reversing the magnetic field applied to saidparticles in successive earth formations to subject said particles tosuccessive magnetic fields in opposite directions during time intervalswhich are long relative to the time intervals for reversing saidmagnetic field, detecting at different times during alternate timeintervals the magnetic relation signal resulting from relaxation of themacroscopic magnetic moment associated with said particles towardalignment with said magnetic fields, and deriving an indication of thecomparative magnitude of said relation signal at said different times.

13. A method for investigating earth formations traversed by a boreholeand containing particles exhibiting paramagnetic properties in thepresence of a magnetic field H applied to said particles, comprising thesteps of periodically reversing the magnetic field applied to saidparticles in successive earth formations to subject said particles tosuccessive magnetic fields in opposite directions during first timeintervals which are long relative to second time intervals for reversingmagnetic field, and during said first time intervals detecting themagnetic relaxation signal resulting from relaxation of the macroscopicmagnetic moment associated with said particles toward alignment withsaid magnetic field, said first time intervals being sulficiently longfor said moments to relax substantially into alignment with saidmagnetic field, and deriving indications of said relaxation signal toprovide a log of said earth formations.

14. A method for investigating earth formations traversed by a boreholeand containing particles exhibiting paramagnetic properties in thepresence of a magnetic field H comprising the steps of applying amagnetic field to said particles in successive earth formations in agiven direction during first time intervals and in an opposite directionduring alternate time intervals which time intervals are long relativeto the time for reversing said magnetic field, and during said timeintervals detecting the magnetic relaxation signal resulting fromrelaxation of the macroscopic magnetic moment associated with saidparticles toward alignment with said magnetic field.

(References on following page) 3,083,335 11 12 References Cited in thefile of this patent Physical Review, vol 93, No. 4, February 1954, pageN D TE 941, Abstracts A7 and A8. U [TE STATES PA NTS Relaxation Effectsin Nuclear Magnetic Resonance 2,561,489 Bloch et al. July 24, 1951 Absorp 1 ption, by Bloembergen et 211., Physical Review,

2,705,790 Hahn 1955 5 vol. 73, No. 7, April 1948, 679-712.

OTHER REFERENCES Measurement of Electronic susceptibilities by MeansBloembcrgen at at Physical Review, VOL 93, 1, of Nuclear ResonanceAbsorption," by Feher et al., Rev. Jam 1, 1954, 2 of Sci. Inst," vol.26, No. 3, March 1955.

Damon: Reviews of Modern Physica, vol. 25, No. 1, Hahn: Physical Review,y 1, 1949,

January 1953, 239-245. 10 pp. 145146;

Bloom et al.: Physical Review, vol. 97, No. 6 pp. 1699- TOIIEYI PhysicalRfiview, 8, 1949,

1709, Mar. 15, 1955. pp. 1059-1066.

Notice of Adverse Decision in Interference In Interference No. 93,939involvin Patent No. 3,083,335, N. A. Schuster, MAGNETIC RESONANCE METHOS AND APPARATUS, final judgment adverse to the patentee was renderedJan. 29, 1970, as to claim 8-.

[Official Gazette July 7, 1970.]

Disclaimer 3,083,335.Niok A. Schuster, Rid -afield, Conn. MAGNETICRESONANCE METHODS AND APPXRATUS. Patent dated Mar. 26, 1963. Disclaimerfiled May 15, 1970, by the assignee, Sohlumberger TechnologyCorporation. Hereby enters this disclaimer to claim 8 of said patent.

[Ofioial Gazette August 18, 1970.]

4. APPARATUS FOR OBSERVING MAGNETIC RELAXATION PHENOMENA IN PARTICLESEXHIBITING PARAMAGNETIC PROPERTIES COMPRISING MEANS FOR APPLYING APOLARIZING MAGNETIC FIELD TO SAID PARTICLES, MEANS FOR PERIODICALLYREVERSING THE DIRECTION OF SAID POLARIZING MAGNETIC FIELD DURINGINTERVALS SHORT WITH RESPECT TO THE RELAXATION TIME T1 ASSOCIATED WITHSAID PARTICLES, AND MEANS FOR DETECTING VARIATIONS IN THE RESULTANTMAGNETIC FIELD IN THE VICINTY OF SAID PARTICLES WHICH ARE A FUNCTION OFMAGNETIC RELAXATION OF SAID PARTICLES DURING THE PERIODS BETWEEN SAIDINTERVALS,
 8. IN A METHOD OF DETERMINING THE MAGNETIC RESONANCEPROPERTIES OF THAT PORTION OF MATERIAL LOCATED AT A PREDETERMINED DEPTHBENEATH THE SURFACE OF SAID MATERIAL, THE STEPS OF APPLYING ASUBSTANTIALLY CONSTANT DIVERGENT MAGNETIC FIELD BETWEEN SPACED-APARTLOCATIONS ON SAID SURFACE WHEREBY SAID FIELD DECREASES IN INTENSITY WITHTHE DISTANCE BEHIND SAID SURFACE, APPLYING AN ALTERNATING MAGNETIC FIELDPERPENDICULARLY TO SAID CONSTANT FIELD IN SAID MATERIAL AND AT AFREQUENCY EQUAL TO THE RESONANCE PRECESSION FREQUENCY OF SELECTEDPARAMAGNETIC PARTICLES IN A CONSTANT FIELD OF THE PARTICULAR INTENSITYOF SAID CONSTANT FIELD AT THE PREDETERMINED DEPTH OF SAID PORTION, ANDOBTAINING INDICATIONS OF THE MAGNETIC RESONANCE IN SAID PARTICLES.